The use of comparators, and their associated voting algorithms, within digital wireless communication systems is known. In general, a comparator, operably coupled to a plurality of base stations or satellite receivers located in geographically diverse areas, attempts to select or construct a favorable representation of an audio signal given multiple sources of the signal (i.e., the base stations). This is accomplished by comparing the signals received from the signal sources and selecting, from amongst all of the signal sources, portions of the signal having the best signal quality. The selected portions are then reassembled to produce the favorable signal representation. The favorable representation can then be retransmitted, thereby increasing the probability of good reception (i.e., intelligibly decoded audio) at the signal destination.
Within such digital communication systems, audio signals are typically represented as streams of compressed digital data. For example, current ASTROTAC.TM. comparators by Motorola, Inc. utilize compressed digital audio parsed into 30 ms. frames, each frame comprising six codewords. Thus, while it is possible to perform the necessary comparisons using well-known quality indicators such as signal-to-noise ratios (SNR), the direct comparison of digital data streams is also possible. As a result, digital comparators (e.g., ASTROTAC.TM.) are known to use codeword voting to determine the signal best suited for retransmission.
FIG. 1 illustrates an example of codeword voting as performed in prior art comparators. As shown, a comparator (101) receives frames (102-103) from N different signal sources (only two shown), such as base stations and/or receivers. The frames (102-103) ideally correspond to identical portions of a transmitted signal. Each of the frames (102-103) comprises six codewords (106-117), identified for clarity as C.sub.ij, where i indicates the signal source and j indicates the codeword position within the frame. Furthermore, each codeword (106-117) also includes a codeword error status, labeled as E.sub.ij. Typically, the codeword error statuses are determined by the base stations/receivers and sent to the compartor. In FIG. 1, it is assumed that each error status represents the number of bit errors detected in their respective codewords (106-117). As a result, a codeword having the lowest error status (i.e., fewer bit errors) is assumed to be the least corrupt and representative of the best possible audio quality.
According to the prior art method, the comparator (101) compares all codewords (106-117), based on their respective codeword error statuses, having equivalent identifications (frame positions). Thus, all codewords C.sub.i1, for i=1 to N, are compared based on their error statuses, E.sub.i1. In the example shown, E.sub.N1 &lt;E.sub.11 and C.sub.N1 is thus selected by the comparator (101) for use in the favorable signal representation (104). Using the same procedure, C.sub.12, C.sub.N3, C.sub.14, C.sub.15, and C.sub.N6 are also selected. This process is repeated each time a new set of frames is input to the comparator (101).
The above-described method works well from an audio quality point of view in that voting occurs on each small segment of the signal (i.e., the codewords). A limitation of this method, however, is the amount of throughput required to send the codeword error statuses to the comparator. For example, 30 ms. frames that include 14 codeword error status bits require a throughput rate of 466.7 bits per second for the codeword error status bits alone. If the frame rate or the number of bits used for the error statuses increases, the required throughput rate is increased. For example, the Association of Public Safety Communication Officers (APCO) has created a standard specifying 20 ms. frames. In order to achieve compliance with this standard, either more throughput capacity is needed to send the same number of error status bits or less error status bits can be sent. Since the throughput rates of the links connecting the signal sources to the comparator (e.g., telephone lines) are typically limited, the first alternative is generally not viable. However, if the number of error status bits per frame is decreased, the resulting codeword error statuses may not be able to provide accurate measurements of each codeword's quality. Therefore, the need exists for a method that allows a digital compartor to provide a favorable signal representation based on fewer error status bits, and yet provide suitable audio quality.